The term 'Underground Hollow' can be broken down into two crossword solutions: 'CAVERN' (6 letters) and 'CAVE' (4 letters). A CAVERN is a large, naturally occurring underground space, often with spectacular formations of stalactites and stalagmites. Caves are smaller than caverns, and are also naturally occurring underground spaces. Both caves and caverns are formed by the erosion of rock and other geological processes.